An MRI device is a medical image diagnosis device that generates magnetic resonance in an atomic nucleus in an arbitrary section that crosses a test object to obtain a tomographical image in the section from a generated magnetic resonance signal. The MRI device transmits a radio frequency wave (hereinafter, referred to as RF) which is a type of electromagnetic wave to the test object, excites spin of the atomic nucleus in the test object, receives a nuclear magnetic resonance signal generated by the nuclear-spin, and images the test object. The transmission of the RF to the test object is performed by an RF transmission coil, and the reception of the nuclear magnetic resonance signal from the test object is performed by an RF reception coil.
In recent years, in order to enhance a signal-to-noise ratio (SNR) of an image, a static magnetic field intensity may tend to increase, and a high magnetic field MRI device (super-high magnetic field MRI device) in which a static magnetic field intensity tends to be strong to be 3 T (teslas) or higher has been spread. However, as the static magnetic field intensity increases, the SNR is enhanced, but unevenness in a captured image easily occurs. This is because the frequency of an RF used for induction of the magnetic resonance phenomenon increases according to the increase in the magnetic field. For example, in an MRI device having a static magnetic field intensity of 3 T (teslas) (hereinafter, referred to as a 3 T MRI device), an RF having a frequency of 128 MHz is used. In a living body, a wavelength of such an RF is about 30 cm which is approximately the same size as that of an abdomen section, and a change occurs in its phase. An emitted RF distribution and a space distribution of a rotating magnetic field that induces the magnetic resonance phenomenon (hereinafter, referred to as a radio frequency magnetic field distribution B1) that is generated by the RF become non-uniform by change in the phase, which makes image unevenness. Accordingly, in RF emission performed in an ultra-high magnetic field MRI device, a technique for reducing non-uniformity of distribution of the rotating magnetic field B1 is necessary.
As an RF emission method for reducing the non-uniformity of the B1 distribution, a method called “RF shimming” is used. This is a method for controlling, using a transmission coil having plural channels, controlling a phase and an amplitude of RF pulses given to each channel to reduce the B1 non-uniformity in an imaging region. The B1 distribution of each channel is measured in advance before main imaging, and an amplitude and a phase of optimal RF pulses for reducing the B1 non-uniformity are calculated using the B1 distribution. Here, a region to be diagnosed which is a partial region in a section is set as a region of interest (ROI), and the amplitude and the phase are determined so that the B1 non-uniformity in the ROI is reduced.
Further, in the MRI device, a specific absorption rate (SAR) of the RF in the living body is regulated to be in a predetermined range in consideration of safety for the living body. A technique for setting RF pulses so that an SAR in the whole living body (hereinafter, a whole body SAR) becomes as small as possible in consideration of the regulation is proposed (for example, see PTL 1). However, in addition to the whole body SAR, an SAR locally generated in the living body (hereinafter, referred to as a local SAR) may also be considered. The whole body SAR may be measured with accuracy to some extent, but it is difficult to measure the local SAR. Accordingly, the local SAR is calculated by data analysis using a numerical simulation, or the like (for example, see NPL 1). Further, as a method for controlling the local SAR, when repeating RF pulses in a multi-shot sequence, a technique for changing a region where energy concentrates, that is, a position of an SAR hot spot by changing each pulse waveform has been proposed (for example, see PTL 2).